Polyester film and method for producing the same

ABSTRACT

The present invention provides a polyester film particularly useful as a base film for a magnetic recording medium such as cassette-type magnetic tape of a digital recording mode and a production process thereof. The polyester film is produced by irradiating UV light onto a surface of a film in a non-stretched stage or a stage prior to the completion of stretching to form fine protrusions on the surface. The fine protrusions on the film surface are specified by the ratio of 10-point average roughness Rz to center line average roughness Ra (Rz/Ra) on the surface (less than 20), the difference in concentration of carboxyl groups between the surface layer part and the inside of the film, and the number of fine protrusions.

TECHNICAL FIELD

The present invention relates to a polyester film having fineprotrusions formed on a surface. More specifically, the presentinvention relates to a polyester film that has good running durabilityand wear resistance and also has extremely good productivity and, whenused as a base material for a magnetic recording medium particularlyhaving a ferromagnetic metal thin film layer thereon, exhibits goodoutput characteristics, and also relates to a process for producing thepolyester film.

BACKGROUND ART

Polyester films can be formed in a continuous manner into large areafilms which cannot be produced with other resin materials. In addition,because of their good properties in strength, durability, transparency,flexibility and surface characteristics, they have been used in thefields needing them in large quantities, such as magnetic recording,industrial, packaging, agricultural and building materials. Among them,biaxially oriented polyester films have been used in various fieldsbecause of their good mechanical, thermal and electric properties andgood chemical resistance. In particular, as base films for magnetictapes, polyester films are unrivaled by other films in usefulness.

In recent years, in the process for processing a polyester film, theprocess for deposition and application of a magnetic layer during themagnetic recording medium production or the process for application of aheat sensitive transfer layer during the thermal transfer materialproduction, it is demanded to increase the processing speed or tofurther improve the quality of a finished product. As these requirementsgrow, it is also required that polyester films have a film surface withmore improved running durability and wear resistance.

To meet the requirements stated above, it is known to be effective touniformly form fine protrusions on a surface of a polyester film. Forexample, a polyester film is known in which substantially sphericalsilica particles such as colloidal silica are contained to provide fineprotrusions on the film surface (e.g., Japanese Unexamined PatentApplication Publication No. 59-171623). A polyester film is also knownin which a thin film layer containing fine particles that providesurface protrusions is laminated on a base layer (e.g., JapaneseUnexamined Patent Application Publication Nos. 62-130848, 2-77431 and8-30958).

On the other hand, magnetic recording media are becoming denser witheach passing year and the wavelength employed for recording is becomingshorter, and the mode of recording is shifting from analogue form todigital form. When a ferromagnetic metal thin film layer is provided onone surface of a base film to produce a magnetic recording medium, theferromagnetic metal thin film layer is usually provided on an ultra-flatfilm surface. In this case, since the ferromagnetic thin film layergenerally has a thickness of as thin as about 0.02 to 0.5 μm, thesurface geography of the base film may be directly reflected as thesurface geography of the finished ferromagnetic thin film. Therefore, itis strongly required to reduce the height of surface protrusions of abase film and form ultra-fine protrusions at high density to providesurface smoothness and surface slipperiness, and development of filmshaving a surface that satisfies these requirements has been demanded.

If ultra-fine particles are contained in the film surface in a largeamount, however, generation of coarse protrusions caused by aggregationof the particles cannot be avoided and it would be difficult to formfine protrusions uniformly and at high density by using particles.Although addition of particles is effective for imparting surfaceslipperiness to the finished film and reducing the friction coefficientbetween the film and a conveyor roll during the film forming/processingprocess, problems still remain that coarse protrusions may drop off ontothe conveyer roll so as to scratch the film and that the outputcharacteristics of a magnetic tape produced using the film may bedeteriorated. Therefore, it is quite difficult to employ theabove-mentioned particle addition method as the means for formingultra-fine protrusions at a high density in the industrial production.

An alternative method is known in which desired fine protrusions areformed on a surface by the action of crystallization of polyesterwithout relying on the particle addition method (e.g., JapaneseUnexamined Patent Application Publication No. 7-1575). According to themethod utilizing the crystallization of polyester, the finished film canhave good running durability and wear resistance since generation ofvoids around the particles can be prevented.

As the techniques for utilizing the crystallization of polyester, thereare known a heat treatment method by winding a polyester film around aheated roll; a heat treatment method with an infrared heater; and amethod of heating with a stenter. However, these methods have seriousproblems of (1) and (2) below. In these conventional heating methods,since the whole film is heated, there is also such a problem thattroubles resulting from slack and adhesion of the film may frequentlyoccur.

(1) It is difficult to stably produce high quality polyester films eachhaving identical quality, since the number of fine protrusions wouldvary depending on apparatus-specific variable factors such as unevennessin temperature during the film formation.

(2) It is impossible to increase the film formation speed, since ittakes much time to form ultra-fine protrusions by crystallization ofpolyester.

Alternatively, it has been employed to irradiate a film with ultravioletlight for modification of chemical properties and patterning of a filmsurface. For example, irradiation with ultraviolet light is employed inthe coating or lamination of a chemical substance (e.g., UV-curableresin) on a surface of a polyester film to improve the adhesion,cohesion, anti-static properties, mechanical properties, opticalproperties and so on or in the patterning of the surface with aphotosensitive resin (e.g., Japanese Unexamined Patent ApplicationPublication No. 11-65130). There is a method in which an ultravioletcurable resin layer containing powder particles is provided on the filmsurface and the curable resin layer is then irradiated with ultravioletlight to form recessed-and-projecting patterns on the surface (e.g.,Japanese Unexamined Patent Application Publication Nos. 11-277451 and10-296944). As a method for chemically modifying the film surface, thereis a method in which active oxygen is generated by the combination ofozone treatment and ultraviolet ray treatment to improve the adhesion ofthe film surface (e.g., Japanese Unexamined Patent ApplicationPublication Nos. 5-68934 and 11-236460).

As mentioned above, irradiation of a film with ultraviolet light hasbeen employed for denaturing a chemical substance applied on the film(e.g., curing of an ultraviolet curable resin layer) or for modifyingthe chemical properties of the film surface (e.g., improvement inadhesion of the surface).

The object of the present invention is to provide a high qualitypolyester film that has a surface superior in running durability andwear resistance, exhibits good output characteristics when used in amagnetic recording medium, and is superior in productivity, processsimplification and production cost, and also to provide a productionprocess for the polyester film.

DISCLOSURE OF INVENTION

The present inventors have made intensive studies for the purpose ofachieving the above object. As a result, it has been found that a methodfor forming surface protrusions by an internal heating system utilizingelectron transition caused by irradiation with ultraviolet light canmake it possible to form fine protrusions on the irradiated surfacestably, so that a polyester film surface can be produced that is farsuperior to that produced by the conventional surface protrusionformation technologies achieved by heat transfer or external radiantheating with hot air, a heated roll or an infrared heater. This findingleads to the accomplishment of the invention.

That is, the production process for a polyester film of the presentinvention is characterized in that at least one surface of a film isirradiated with ultraviolet light to form fine protrusions on thesurface.

Since the irradiation with ultraviolet light enables to heat the filmsurface more selectively, the formation of fine protrusions on thepolyester surface can be achieved quite readily. By the irradiation withultraviolet light, a higher-order structure of the polyester can beformed. As a results fine protrusions can be formed stably andefficiently and a polyester film can be produced with good productivitythat has surface properties good in wear resistance and runningdurability and suitable for use in a magnetic recording medium.

Using the film according to the present invention as a base film, amagnetic recording medium having good output characteristics can beproduced stably.

The production process for a polyester film of the present inventionencompasses the following desirable embodiments.

(a) The irradiation with ultraviolet light is performed with a lightsource having a relative intensity of light with wavelengths of 270 to300 nm of 10% or more and containing substantially no wavelength shorterthan 250 nm.

(b) The energy density employed during the ultraviolet light irradiationis 0.1 to 10 J/cm² and the irradiation time is 0.01 to 100 seconds.

(c) After irradiating at least one surface of the film with ultravioletlight, the film is stretched in a longitudinal direction and/or atransverse direction.

The film produced by the present process is characterized in that: (1)fine protrusions are present on at least one surface of the film, theratio of 10-point average roughness Rz to center line average roughnessRa (Rz/Ra) on the surface is less than 20, and the concentration ofcarboxyl groups in the surface layer part of a thin layer which providesthe surface is greater than that in the inside of the thin layer; and/or(2) a film surface portion that contains no protrusion with a protrusionheight of 10 nm or more constitutes at least 5% of the whole filmsurface, and the number of protrusions with a protrusion height of notless than 3 nm and less than 5 nm in the film surface portion is notless than 1×10⁶/mm² and less than 1×10⁹/mm².

The polyester film encompasses the following desirable embodiments.

(a) The difference in concentration of carboxyl groups between thesurface layer part of a thin layer which provides the surface havingfine protrusions thereon and the inside of the thin layer is 0.001 ormore.

(b) In the film surface portion that does not contain protrusions with aprotrusion height of 10 nm or larger, the grain size at the protrusionheight threshold value of 3 nm is not less than 1 nm² and less than 5000nm².

(c) Particles with a particle diameter as mono-disperse particles and/ora primary particle diameter as aggregate particles of not less than 1 nmand less than 300 nm are contained in an amount of not less than 0.01%by weight and less 1% by weight.

(d) The film is a laminated film that is composed of A layer mainlycomposed of polyester A laminated on at least one surface of B layermainly composed of polyester B.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinbelow, the present invention is illustrated in detail.

As used herein, the polyester refers to a polymer produced bycondensation polymerization of a diol and a dicarboxylic acid. Thedicarboxylic acid is typically terephthalic acid, isophthalic acid,phthalic acid, naphthalenedicarboxylic acid, adipic acid, sebacic acidor the like. The diol is typically ethylene glycol, trimethylene glycol,tetramethylene glycol, cyclohexane dimethanol or the like. Examples ofthe polyester include polymethylene terephthalate, polyethyleneterephthalate, polypropylene terephthalate, polyethylene isophthalate,tetramethylene terephthalate, polyethylene-p-oxybenzoate,poly-1,4-cyclohexylene dimethylene terephthalate andpolyethylene-2,6-naphthalate.

The polyester may be a homopolymer or copolymer. As a copolymerizablecomponent, a diol component such as diethylene glycol, neopentyl glycol,and polyalkylene glycol; and a dicarboxylic acid component such asadipic acid, sebacic acid, phthalic acid, isophthalic acid and2,6-naphthalenedicarboxylic acid, for example, may be used.

Besides the above mentioned dicarboylic acid components and diolcomponents, aromatic hydroxycarboxylic acids such as p-hydroxybenzoicacid, m-hydroxybenzoic acid, 2,6-hydroxynaphtoic acid, p-aminophenol andp-aminobenzoic acid may be copolymerized in such a small amount that theadvantages of the present invention are not hindered.

In the polyester film of the present invention, a heterogeneous polymerother than the polyester used as the matrix may be blended as long asthe advantages of the present invention are not hindered. Theheterogeneous polymer is preferably blended in an amount of 0.1 to 30%by weight, more preferably 0.5 to 15% by weight, most preferably 1 to10% by weight, based on 100% by weight of the polyester. Preferredexamples of the heterogeneous polymer include a polyimide, a polyetherimide, a copolymerized polyester containing a mesogen group (a liquidcrystal substituent) in the main chain, a polycarbonate and astyrene-based polymer with a number average molecular weight of 20000 orless.

A blend polymer prepared by appropriately blending the above-mentionedpolyesters may also be used, such as a blend of polyethyleneterephthalate and polyethylene-2,6-naphthalate.

The intrinsic viscosity of the polyester film of the present inventionis preferably 0.55 to 1.0. The intrinsic viscosity is more preferably0.60 to 0.9, and most preferably 0.65 to 0.8. If the intrinsic viscosityof the film exceeds 1.0, fine protrusions may not be formed on thepolyester surface. If the intrinsic viscosity is less than 0.55, thebreakage of the film may frequently occur during the film formation.

As used herein, the ultraviolet light (hereinafter, referred to as UVlight) means light containing light with wavelengths of not longer than400 nm. Particularly, it is preferred to irradiate light withwavelengths of 270 to 330 nm selectively. The light source used for thispurpose is preferably a device capable of irradiating light having arelative intensity of light of 270 to 300 nm in wavelength of not lessthan 10%. More preferably, light with wavelengths of shorter than 250 nmis substantially cut and is not contained. In other words, it ispreferred to use a light source capable of irradiating light having arelative intensity of light of shorter than 250 nm in wavelength of lessthan 1%. When the relative intensity of light of 270 to 300 nm inwavelength is less than 10%, fine protrusions may not be formed on thefilm surface if the energy density during the irradiation is notincreased and it would take much time for formation of protrusions,which is also disadvantageous in the economical viewpoint. If light withwavelengths of less than 250 nm is irradiated onto the film, thephoto-degradation of the polyester would increase and the wearresistance of the film surface would be frequently degraded, whichshould be kept in mind. The relative intensity of light with wavelengthsof 270 to 300 nm is preferably 25% or greater, and more preferably 35%or greater.

As the light source used in the present invention, for example, a lampsuch as a high pressure mercury lamp and a metal halide type of lamp anda laser beam irradiation device can be preferably used. Particularlypreferred is a metal halide type of light source.

When a lamp is used as the light source, the mode for irradiating lightmay be of any type, such as converging type, parallel (semi-converging)type and diffusing type, and may be properly selected depending on thecomposition of the polymer used, the production conditions, the facilityused for the production and the like.

When a laser beam irradiation device is used, the type of the device isnot particularly restricted, but a device capable of irradiating a laserbeam with wavelengths of 270 to 330 nm is particularly effective.

For the irradiation with UV light in the present invention, it ispreferred to selectively use light with wavelengths of 270 to 330 nm.Therefore, it may also be preferred to use various optical filters incombination. The optical filter includes, for example, an opticalcoherence filter, a bandpass filter, a short wavelength cutting filter,a long wavelength cutting filter and a absorptive material such asquartz glass and a colored glass.

In the present invention, it is preferred to irradiate with UV lighthaving an energy density of 0.1 to 10 J/cm² for an irradiation time of0.01 to 100 seconds. As used herein, the energy density refers to anintegrated value which is measured with a UV meter equipped with asensor capable of detecting light of 300 to 390 nm in wavelength.

If the energy density is less than 0.1 J/cm² or the irradiation time isshorter than 0.01 second, then fine protrusions are not likely to beformed. On the contrary, if the energy density exceeds 10 J/cm² or theirradiation time exceeds 100 seconds, then deterioration of the surfacewould be increased and the wear resistance would be degraded.

The more preferred irradiation conditions include the energy density of0.2 to 5 J/cm² and the irradiation time of 0.1 to 20 seconds. The stillmore preferred irradiation conditions include the energy density of 0.4to 3 J/cm² and the irradiation time of 0.2 to 10 seconds.

In the present invention, after irradiating the film surface with UVlight, it is preferred to stretch the film in a longitudinal directionand/or a transverse direction. Particularly preferably, the irradiationwith UV light is performed prior to stretching a biaxially orientedpolyester film in a longitudinal direction and/or a transversedirection. As used herein, the biaxially oriented polyester film refersto a polyester film oriented both in a transverse direction and amachine direction. The transverse direction and the machine directionmean a longitudinal direction and a transverse direction of the film,respectively.

In the polyester film of the present invention, the degree oforientation fn of the film surface is preferably 0.08 to 0.20, morepreferably 0.01 to 0.19, in view of the scratch resistance.

The film to be irradiated with UV light may be a non-stretched filmproduced by the extrusion/casting process or a film produced bystretching the non-stretched film in a transverse direction and/or amachine direction. Among these, a non-stretched film, a slightlyoriented film which has been slightly stretched in a transversedirection, or a monoaxially stretched film which has been stretched in atransverse direction is preferred for the irradiation with UV light. Itis most preferred to irradiate a non-stretched film with UV light.

In the polyester film of the present invention which is produced by theabove-mentioned UV light irradiation method, the ratio of 10-pointaverage roughness Rz to center line average roughness Ra (Rz/Ra) on atleast one surface of the film is less than 20; and the concentration ofcarboxyl groups in the surface layer part of a thin layer which providesthe surface is greater than that in the inside of the thin layer. If theratio of 10-point average roughness Rz to center line average roughnessRa (Rz/Ra) is 20 or more, then the film surface protrusions would benonuniform in height. As a result, wear resistance of the film would beunsatisfactory and, when used in a magnetic tape, output characteristicsof the magnetic tape would also be unsatisfactory.

To improve the wear resistance of the film and the outputcharacteristics of the magnetic tape produced using the film by theformation of fine protrusions on the film surface, it is preferred thatthe surface roughness Rz/Ra value be lower. However, it is quitedifficult to produce a film with a Rz/Ra value of less than 2 in theindustrial scale and, if produced, the productivity of the film is oftenpoor. Therefore, the lower limit of the Rz/Ra value is preferably 2. Theratio of 10-point average roughness Rz to center line average roughnessRa (Rz/Ra) on at least one surface of the film is preferably less than15, more preferably less than 10.

The center line average roughness Ra of the surface is preferably 0.3 to200 nm, more preferably 0.4 to 100 nm, still more preferably 0.5 to 30nm.

In the film of the present invention, to achieve the intended filmproperties of the present invention, it is required that theconcentration of carboxyl groups in the surface layer part of a thinlayer which provides the surface having fine protrusions thereon begreater than that in the inside of the thin layer. The difference inconcentration of carboxyl groups between the surface layer part of athin layer which provides the surface having fine protrusions thereonand the inside of the thin layer (concentration of carboxyl groups inthe surface layer part-concentration of carboxyl group in the inside) ispreferably not less than 0.001 and less than 0.020, more preferably notless than 0.003 and less than 0.015. If the difference in concentrationof carboxyl groups between the surface layer part and the inside of thethin layer is 0.020 or greater, then the surface degradation would occurand therefore wear resistance would be deteriorated, which should bekept in mind.

In the film of the present invention, a film surface portion thatcontains no protrusion with 10 nm or larger in height constitutes atleast 5%, preferably at least 10%, of the whole film surface, and thenumber of protrusions with not lower than 3 nm and lower than 5 nm inprotrusion height on the film surface portion is not less than 1×10⁶/mm²and less than 1×10⁹/mm², preferably not less than 2×10⁶/mm² and lessthan 5×10⁸/mm², more preferably not less than 5×10⁶/mm² and less than8×10⁷/mm². Although it is industrially possible to form only protrusionswith less than 10 nm in height on the whole surface, the upper limit ofthe ratio of the film surface portion containing no protrusion with lessthan 10 nm in height is preferably 95% or less. If the ratio does notfall within the range, running durability of the film would become poor,problems of deterioration in handling properties and generation of filmssurface defects (e.g., scratches) would occur during the filmformation/processing process, and a problem of contamination during theprocess would also occur.

In the film surface portion that contains no protrusion with 10 nm orgreater in thickness (as determined using an atomic force microscope(AFM) at the field of view of 0.5 μm×0.5 μm), the grain size at theprotrusion height threshold value of 3 nm is preferably not less than 1nm² and less than 5000 nm².

If the number of surface protrusions with not less than 3 nm and lessthan 5 nm in protrusion height on the film surface portion containing noprotrusion with 10 nm or greater in height does not fall within theabove-mentioned range, it is not preferable since it would be difficultto achieve good running durability or scratch resistance. The formationof fine protrusions greatly in number is further desirable, since thefriction between the film and an object to be contact with the film canbe reduced and, therefore, scratch resistance of the film and outputcharacteristics of a magnetic tape produced using the film can befurther improved.

In the film of the present invention, when the number of the protrusionswith 3 nm or more in height is determined under the enlarged field of 5μm×5 μm to take an AFM image in the above-mentioned AFM method, it ispreferred that the number of the protrusions be 2×10³ to 1×10⁸protrusions/mm² More preferably, the number of the protrusions is 2×10⁴to 5×10⁷ protrusions/mm² in view of running durability and scratchresistance.

Since the fine protrusions on the film surface formed by irradiationwith UV light of the present invention are not principally those formedaround core particles added as seeds, they have lower hardness comparedwith fine protrusions formed around core particles. The fine protrusionsformed by the present process are relatively soft. Therefore, even whenthe film is run on a plastic guide, the scraping of the guide is reducedand a problem resulting from the scraping of the guide surface can beeliminated. Since the protrusions are relatively soft and the protrusionheight is uniform, a problem of wearing of a MR head (MagneticResistance Head) utilizing magnetic resistance effect for use inmagnetic recording media can also be eliminated.

Whether the protrusions on the film surface formed by irradiation withUV light of the present invention are those induced from core particlesadded as seeds (hereinafter, also referred to as “core particle-inducedprotrusions”) or not can be determined by the following method, and itis preferred that the percentage of the protrusions not induced fromcore particles be at least 70%.

A film is etched with a suitable solvent to remove the beneath portionsof protrusions to be determined in the direction of film thickness. If amaterial that constitutes the protrusions remains as an insolublematter, then the protrusions are presumed as those induced fromparticles added externally or deposited internally (I). If no insolublematter remains, then the protrusions are presumed as those not inducedfrom particles (II). The solvent that is preferably used for thispurpose may be a mixed solvent such as phenol/carbon tetrachloride(weight ratio: 6:4). The frequencies of (I) and (II) are determined bythis method with the field of view of about 1 mm². The percentage of theparticle-induced protrusions is expressed by (II)/[(I)+(II)].

Alternatively, for determining whether the surface protrusions are thoseinduced from particles or not, a method may also be employed in which anultra-thin sectional slice of a film is observed with a transmissionelectron microscope (TEM) and a protrusion whose length taken in thedirection of film thickness exceeds the average film surface protrusionheight is presumed as a protrusion induced from a particle. The methodfor determining whether the surface protrusions are those induced fromparticles or not is not restricted to those methods mentioned above andother suitable method may be employed.

In the polyester film of the present invention, it is not necessary toadd particles in view of the surface protrusion formation. However,inorganic or organic particles, other various additives such asantioxidizing agents, antistatic agents and nucleating agents may beadded, as long as the advantages of the present invention are nothindered.

The inorganic particles include, for example, particles of an oxide suchas silicon oxide, aluminum oxide, magnesium oxide and titanium oxide; acomposite oxide such as kaolin, talk and montmorillionite; a carbonatesuch as calcium carbonate and barium carbonate; a sulfate such ascalcium sulfate and barium sulfate; a titanate such as barium titanateand potassium titanate; a phosphate such as calcium phosphate tribasic,calcium phosphate dibasic and calcium phosphate monobasic; or the like,but are not restricted to these compounds. Two or more of thesecompounds may be used in combination depending on the intended use.

The organic particles include, for example, particles of polystyrene orcrosslinked polystyrene, crosslinked particles of stylene-acrylate or anacrylate, vinyl-based particles such as those of styrene-methacrylate ora methacrylate, particles of benzoguanamine-formaldehyde, silicone orpolytetrafluoroethylene or the like. However, they are not restricted tothese compounds and any particles may be used as long as at leastportion of the particles are organic polymer microparticles insoluble inpolyester.

When these particles are contained, the particle diameter in the form ofmonodisperse particles and/or the primary particle diameter in the formof aggregate particles is not less than 1 nm and less than 300 nm,preferably not less than 5 nm and less than 200 nm, and still morepreferably not less than 10 nm and less than 100 nm. If the particlediameter is less than 1 nm, then the particles are likely to aggregateand coarse protrusion may be formed. If the particle diameter is greaterthan 300 nm, then it is difficult to achieve good scraping resistance ofthe film and output characteristics of a magnetic tape produced usingthe film. The content of the particles is not less than 0.01% by weightand less than 1% by weight, preferably not less than 0.05% by weight andless than 0.5% by weight. If the content of the particle exceeds 1% byweight, then it is not only difficult to produce the film of the presentinvention but also generation of coarse protrusion is likely to occurdue to the aggregation of the particles and, therefore, it would becomedifficult to achieve satisfactory scraping resistance of the film andoutput characteristics of a magnetic tape produced using the film.

When the film is a monolayered polyester film, fine protrusions may beformed by effecting the irradiation with UV light according to thepresent invention only in the extreme surface layer part of the film.Accordingly, although the film of the present invention may be amonolayered film, the film is preferably of a laminated structure forthe purpose of forming fine protrusions greatly in number or formingdifferent types of surface protrusions on the surface and the backsurface of the film. When the film has a laminated structure, the thinfilm of the surface means a laminated layer part on the film surface,whereas when the film has a monolayered structure, it means the wholefilm.

When the film has a laminated structure, it is preferred to laminate apolyester layer (A layer) on which fine protrusions are formed by theirradiation with UV light onto at least one surface of other polyesterlayer (B layer). For an A/B two-layered structure, the B layer maycontain particles or not, but it is preferred to contain particles inview of handling of the film and winding properties. When particles arecontained in both A layer and B layer, it is preferred that B layercontain larger particles than those contained in A layer.

When the film of the present invention is used as a base film for amagnetic recording medium, it is desirable that a magnetic layer beprovided on the surface where the surface protrusions are formed byirradiation with UV light (i.e., the surface of the A layer side whenthe film has a laminated structure), but is not particularly restricted.

In the present invention, it preferred that the difference incrystallinity parameter between the surface layer part of at least onesurface and the central layer part of the film as determined by Ramanspectrometry be 1.0 or less. If the difference in crystallinityparameter determined by Raman spectrometry exceeds 1.0, then the amountof curls generated during the storage in the form of a roll under hightemperature/high humidity conditions is likely to increase and thesurface evenness of the film is likely to become poor. For a magnetictape application, poor surface evenness may cause undesirable adhesionbetween the tape and a head, resulting in deterioration of outputcharacteristics.

The whole thickness of the film of the present invention may be suitablyspecified depending on the application purpose and the intended use. Formagnetic material application, the whole thickness is preferably 1 to 20μm in general. Particularly, it is preferred that the whole thickness be2 to 9 μm for high density magnetic recording coating-type mediumapplication and be 3 to 9 μm for high density magnetic recordingdeposition-type medium application. For floppy disk application, thewhole thickness is preferably 30 to 100 μm. For industrial materialapplication, the whole thickness is preferably 1 to 6 μm for thermaltransfer ribbon application, 0.5 to 10 μm for electric capacitorapplication and 0.5 to 5 μm for heat sensitive mimeograph stencilapplication.

Hereinbelow, the production process for a polyester film is explainedmore in detail. However, the present invention is not limited by thefollowing explanation of production examples.

The polyester used in the present invention may be any one produced bythe conventional method. When particles are contained in the polyester,the particles may be added at any stage, for example, prior topolymerization, during polymerization or after polymerization. However,it is preferred to add the particles by dissolving the particles in adiol component (e.g., ethylene glycol) to obtain a slurry and thenmixing and dispersing the slurry in the polyester prior to completion ofthe polymerization to thereby polymerize the ethylene glycol with apredetermined dicarboxyl acid component. To further enhance theadvantages of the present invention, it is effective to heat-treat theslurry of the particles in ethylene glycol at a temperature of 150 to230° C., particularly 180 to 210° C., for 30 minutes to 5 hours,preferably 1 to 3 hours.

As the method for controlling the content of the particles added, it iseffective to employ a method of controlling the content by distillinghigh concentration particle master pellets with a polymer thatsubstantially contains no particle during the film formation.

Hereinbelow, the production of an A/B two-layered laminated polyesterfilm that is composed of A layer composed of polyester A and B layercomposed of polyester B is illustrated as an example. The polyester usedin the present invention is preferably polyethylene terephthalate (PET)mainly composed of ethylene terephthalate. PET may be produced by directpolymerization method or DMT method. For the production by DMT method,it is preferred to use calcium acetate as a transesterificationcatalyst. During the polymerization step, it is also preferred to use(but not limited to) a germanium compound as a polymerization catalyst.As the germanium catalyst, as known in the art, the following materialsmay be used: (1) amorphous germanium oxide, (2) crystalline germaniumoxide with 5 μm or smaller, (3) a solution prepared by dissolvinggermanium oxide in glycol in the presence of an alkali metal or analkaline earth metal and (4) a solution of germanium oxide in glycolprepared by dissolving germanium oxide in water, adding glycol to thesolution and then evaporating the solution to remove water.

The solution haze of the polyester may be 5% or less, preferably 3% orless, more preferably 1% or less. If the solution haze exceeds 5%, theamount of deposited particles and particles added in the solution wouldbe increased and, therefore, a surface desired in the present inventionmay not be formed and scraping resistance may be degraded.

The raw materials for polyester A and polyester B are separately driedin vacuo at 180° C. for at least 3 hours, supplied respectively to twosingle- or twin-screw extruders which has been heated to 270 to 310° C.under nitrogen gas stream or in vacuo so that the intrinsic viscosity isnot decreased, and then extruded from a T die as a sheet. Polyester Aand polyester B are laminated together in a polymer tube or a die.

Subsequently, the molten laminated sheet is electrostatically caused toclosely contact with a drum that has been cooled at a surfacetemperature of 10 to 40° C. and cooled to solidify, thereby obtaining asubstantially amorphous non-stretched laminated film. To enhance theadvantages of the present invention, it is preferred to control thelamination thickness of each layer by controlling the amount of apolymer extruded by means of a static mixer and a gear pump provided inthe polymer pass.

The resultant non-stretched film is irradiated with UV light having anenergy density of 0.1 to 10 J/cm² for an irradiation time ranging from0.01 to 100 seconds. As stated above, the irradiation with UV light maybe performed after obtaining the non-stretched film, after stretchingthe film slightly or after stretching the film in a transverse and/ormachine direction. In the present invention, it is preferred toirradiate the non-stretched film. The atmosphere employed for theirradiation with UV light may be under room temperature conditions orheated conditions of 50 to 200° C. In the present invention, it ispreferred to irradiate with UV light under room temperature conditions,in view of good productivity such as simplification of the process.

Subsequently, if required, the non-stretched film is biaxially stretchedto cause the biaxial orientation. As the stretching method, successivebiaxial stretching method or simultaneous biaxial stretching method maybe employed. To produce the film of the present invention withoutbreakage, it is effective to use successive biaxial stretching method inwhich a film is first stretched in a longitudinal direction and then ina transverse direction. The stretching in a longitudinal direction isusually performed using a roll, and the stretching temperature is 80 to180° C., preferably 90 to 150° C. The stretching in a longitudinaldirection is preferably performed in one step or multiple steps (two ormore steps) at a temperature higher than the glass transitiontemperature Tg of polyester A by 15° C. or more at a stretching ratio of2 to 8 times, preferably 2.5 to 7 times, whereby the film of the presentinvention can be produced readily.

The stretching in a transverse direction is preferably performed using aknown tenter at a stretching temperature of 90 to 160° C., preferably100 to 150° C. at a stretching ratio of 2.5 to 6 times, preferably 3 to5 times at a stretching rate of 3000 to 30000%/min. Subsequently, thestretched film is heat-treated. The heat treatment may be performed at atemperature of 180 to 250° C., particularly 200 to 240° C. for 1 to 20seconds. Subsequently, after intermediately cooled at 100 to 180° C.,the film is cooled to room temperature and rolled up, if necessary,while relaxing in a transverse and/or machine direction, whereby thedesired biaxially oriented polyester film can be produced. If it isdesired to increase the strength of the film in a transverse or machinedirection, it is preferred to stretch the film again in atransverse/machine direction prior to the heat treatment. In this case,preferred stretching conditions include a stretching temperature of 110to 150° C. and the stretching ratio of 1.1 to 1.8.

In the production example mentioned above, the production of the filmusing a successive twin-screw extruder is illustrated as an example.However, the film may be produced using a simultaneous twin-screwextruder. In this case, it is preferred to use a stretching apparatus ofwhich driving mode for a clip is a linear motor mode.

[Methods for Evaluation of Physical Properties]

(1) Relative Intensity of Light with 270 to 300 nm in Wavelength (%)

The emission spectrum of light from a light source (wavelength (nm) vs.luminescence intensity (mJ)) is measured using a spectroscope underconditions of 25° C., 60 RH and 1 atm. The data of the emission spectraobtained were analyzed and the relative intensity of light with 270 to300 nm in wavelength was determined according to the following equation.Relative intensity=[(integrated value of luminescence intensity of270–300 nm in emission spectra)/maximum luminescence intensity]×100(%)

The maximum luminescence intensity refers to a luminescence intensity ofwavelength which shows the maximum intensity in the emission spectra.For a UV lamp preferably used in the present invention, the maximumluminescence intensity is a luminescence intensity of 365 nm or 254 nm.

(2) Energy Density of UV Light (J/cm²)

An integral value was determined using an UV meter manufactured by JapanStorage Battery Co., Ltd. (UV350N model), and an integrated value wasdetermined.

(3) Concentration of Carboxyl Groups in a Film

Measurement was made using ESCA according to the method described in theliterature by Nakayama et al. (Y. Nakayama et al., “Surface andInterface analysis”, vol. 24, 711 (1996)). The apparatus and conditionsused for the measurement are given below. In a measurement sample,carboxyl groups were vapor-phase labeled with trifluoroethanol. Thebinding energy was adjusted so that the peak C1s value became 284.6 eV.The concentration of carboxyl groups was calculated as a ratio relativeto the number of carbon atoms in the detection depth.

For the determination of the concentration of carboxyl groups in theinside of the film of the thin layer, the measurement was made by addinghexafluoroisopropanol dropwise onto the film surface on a spin coater todissolve the film surface or, alternatively, shaving a portion of thesurface layer part with a razor to reduce the thickness of the thinlayer to ½ of the original thickness and then doing the above-mentionedcarboxyl group labeling.

(Measuring Device)

-   -   Apparatus body: SSX-100 (manufactured by SSI, USA)    -   X ray source: Al-Kα (10 kv, 20 mA)        (Conditions for Measurement)    -   Degree of vacuum: 5×10⁻⁷ Pa    -   Photo escape angle: 35°        (4) Surface Roughness Ra, Rz of a Film

The center line average roughness Ra and the 10-point average roughnessRz (both were measured in nm) were measured by use of a high precisionthin film gap measuring instrument ET-10 produced by Kosaka Kenkyusho.The measurement conditions are given below. Twenty cycles of measurementwere conducted while the film was being scanned in a transversedirection, and then the resultant values were averaged.

(Conditions for Measurement)

-   -   Radius of feeler tip: 0.5 μm    -   Load of feeler: 5 mg    -   Length of measurement: 0.5 mm    -   Cut-off value: 0.08 mm        (5) Determination of the Ratio of a Film Surface Portion        Containing no Protrusion with 10 nm or Greater in Height        Relative to the Whole Film Surface (One Surface of the Film)

An atomic force microscope (AFM) was used to determine the film surfaceunder the conditions given below.

(Conditions for Measurement)

-   -   Device: NanoScope III AFM (manufactured by Digital Instruments)    -   Canti lever: Silicon single crystal    -   Mode of scanning: Tapping mode    -   Scanning area: 0.5 μm×0.5 μm    -   Scanning rate: 0.5 Hz

An AFM image with a field of 0.5 μm×0.5 μm was taken randomly 100 times.The ratio of frequency of the images having no protrusion with 10 nm orgreater in height was determined as the ratio of the film surfaceportion having no protrusion with 10 nm or greater in height relative tothe whole film surface (%).

(6) Number of Protrusions with not Less than 3 nm and Less than 5 nm inHeight in the Film Surface Portion Having no Protrusion with not Lessthan 10 nm in Protrusion Height.

Among the AFM images taken in the measurement of (5) above, AFM imageshaving no protrusion with 10 nm or greater in protrusion height wereused. The number of protrusions with 3 nm or greater in protrusionheight and the number of protrusions with 5 nm or greater in protrusionheight were counted using a protrusion height threshold value of 3 nmand 5 nm in the field of 0.5 μm×0.5 μm, respectively. The number ofprotrusions with 5 nm or greater in protrusion height was subtractedfrom the number of protrusions with 3 nm or greater in protrusionheight, and the resultant values were averaged and then converted to thenumber of protrusions per 1 mm².

(7) Grain Size at a Threshold Value for Protrusion Height of 3 nm

Among the AFM images taken in the measurement in (5) above, AFM imageshaving no protrusion with 10 nm or greater in protrusion height wereused. The grain size was determined using a protrusion height thresholdvalue of 3 nm and the resultant values were averaged.

(8) Number of Protrusions with 3 nm or Greater in Height as Determinedby AFM Measurement Under the Observation Field of 5 μm×5 μm

AFM images were taken in the same manner as in (5) above, except thatthe scanning area in the AFM measurement conditions was changed to 5μm×5 μm (protrusions with 10 nm or greater in protrusion height might becontained). The measurement of protrusions with 3 nm or greater inheight was performed 20 times, the resultant values were averaged, andthen the resultant values were averaged and then converted to the numberof protrusions per 1 mm².

(9) Crystallization Parameter (ΔTcg)

Measurement was made with DSC (differential scanning calorimeter) IImodel manufactured by Perkin-Elmer. The conditions for measurement aregiven below. Ten mg of a sample was set in a DSC device and melted at300° C. for 5 minutes, followed by quenching of the melt in liquidnitrogen. The resultant specimen was heated at 10° C./min. and checkedin respect of its glass transition point Tg.

Temperature rise was continued, and a crystallizing exothermic peaktemperature derived from a glass state was measured as a coldcrystallization temperature (Tcc); an endothermic peak temperaturederived from crystal fusion was measured as a fusion temperature (Tm);and a crystallizing exothermic peak temperature derived from duringtemperature drop was measured as a crystallization temperature intemperature drop (Tmc).

The difference between Tcc and Tg (Tcc−Tg) is defined as thecrystallization parameter (ΔTcg).

(10) Intrinsic Viscosity of a Film

The intrinsic viscosity was determined as a value measured at 25° C. ata concentration of 0.1 g/ml in ortho-chlorophenol. The unit wasexpressed in [dl/g].

(11) Solution Haze of a Polymer

Two grams of a polyester was dissolved in 20 ml of a mixed solventphenol/carbon tetrachloride (weight ratio of 6/4) and the solution hazewas determined according to ASTM-D-1003-52 using a light pass of 20 mm.

(12) Crystallinity Parameter by Raman Spectroscopy

The crystallinity was evaluated in the surface layer part and thecentral layer part of the film with a laser Raman microprobe.

The apparatus and conditions used for the measurement are given below.

The surface layer part refers to a layer part lying below the filmsurface at a depth of 1 μm, and the central layer part refers to a layerpart lying at a depth of about ½ of the film thickness±0.5 μm.

A film sample to be measured was embedded in an epoxy resin, the crosssection was polished, and the surface layer part and the central layerpart were measured on Raman spectra (n=5). The half band width of 1730cm⁻¹ which corresponded to the stretching vibration of carboxyl wasdefined as a parameter of crystallinity.

A smaller parameter value means a higher crystallinity of a film.

-   -   Device for measurement: Rmanor U-1000 (Jovin-Yvon)        -   Microprobe: Olympus BH-2 model        -   Objective lends: 100×

-   Light source: Argon ion laser (5145A)

-   Detector: PM: RCA31034/Photon Counting System

-   Conditions for measurement:

Conditions for measurement: SLIT 1000 μm LASER 100 mW GATE TIME 1.0second SCAN SPEED 12 cm⁻¹/min SAMPLING 0.2 cm⁻¹ INTERVAL REPEAT TIME 6(13) Average Particle Diameter of Particles

A polyester is removed from a film, by means of plasma ashing, wherebyparticles were caused to expose from external view. The conditions forthis procedure are selected so that the polymer is ashed but theparticles can be protected almost completely from being impaired.Observation is made of the particles by a scanning electron microscope(SEM), and the resultant particle images are treated by an imageanalyzer. The magnification of SEM is set to be approximately 2000 to30000 times, and the field in single measurement is chosen from about 10to 50 μm in one side. In terms of 5000 or more particles in numberobserved at varied locations, the volume average diameter d isdetermined by the particle diameter and volume fraction.

When the particles are of an organic nature or the like and are apt tobecome greatly impaired due to plasma ashing at low temperature, thefollowing method may be employed.

The film is observed in cross section by a transmission electronmicroscope (TEM) at a magnification of 3000 to 400000 times. Thethickness of a slice for TEM inspection is set at about 100 nm andmeasured at a field of 500 or more at varied locations. The volumeaverage particle diameter d is obtained in the same manner as above.

(14) Content of Particles

Compositional analysis was made by means of the microscopic FT-IR method(Fourier's transformation microscopy infrared spectroscopy). The contentof particles was determined based on the ratio of peak arising from acarbonyl group in a polyester to peak arising from materials other thanthe polyester. In order to convert the peak height ratio to thecorresponding weight ratio, the ratio of polyester weight to a totalweight of polyester plus other materials was determined from acalibration curve prepared in advance with use of samples of knownweights. An X-ray microanalyzer may also be employed, if required. Inthe case where a solvent can be selectively used which dissolve apolyester but does not dissolve a particle material, the polyester wasdissolved, and the particles were separated by centrifugally from thepolyester. Thus, the weight percentage of the particle material wasdetermined.

(15) Thickness of Film Laminate

A laminated film is observed cross-sectionally at an acceleratingvoltage of 100 kV with use of a transmission electron microscope (H-600model, manufactured by Hitachi Ltd.) and by means of an ultra slicingmethod (RuO₄ dyeing). The interface of the laminate is captured, fromwhich the thickness of the laminate is determined. Magnifications arenot particularly restricted since they are usually chosen depending onthe thickness of laminates to be measured. However, 1 tens of thousandsto 10 tens of thousands are suitable. When the laminate interface ishardly recognized, a depth distribution of inorganic ions is determinedby means of a secondary ion mass spectrometer. The maximum value in thedirection of depth is determined on the basis of the surface, and adepth found equivalent to ½ of the maximum value is taken as thethickness of the laminate.

(16) Scratch Resistance

Scratching test was made by means of a continuous loading scratchresistance tester HEIDON-18 under the conditions given below and thedepth of scratches were determined with a non-contact roughness meterTOPO-3D manufactured by WYKO.

[Conditions for Evaluation]

-   -   Scratching needle: made of sapphire

Radius of curvature of tip 200 μm Loading: 50 g Running rate: 10 cm/min

Loading:

Running Rate:

On the basis of the depth of scratches, the films were ranked asfollows.

-   -   Scratches with 0.5 μm or less in depth: excellent    -   Scratches with 0.5 to 2 μm in depth: good    -   Scratches with 2 μm or greater in depth: unacceptable        (17) Stability of Surface Protrusion Formation

The state of surface protrusion formation was evaluated by determining asurface roughness Ra as measured by the method in (4) above and thenumber of surface protrusions as determined by the method in (8) aboveat 10 locations in a transverse direction and 30 locations in alongitudinal direction of the film as a 10 cm spacing, and the state ofthe film was ranked on the following scales on the basis of thevariation in determined values.

-   -   ◯: almost no variation was observed in both surface roughness Ra        and number of surface protrusions, and the surface quality was        stable;    -   Δ: a variation by about 20 to 40% was observed in either surface        roughness Ra or number of surface protrusions; and    -   x: a variation by 40% or more was observed in either surface        roughness Ra or number of surface protrusions.        (18) Output Characteristics of ME Tape

An ME tape was measured on C/N at 7 MHz±1 MHz using a commerciallyavailable VTR device for Hi 8 (EV-BS3000, manufactured by SonyCorporation). The C/N value thus obtained was compared to that of acommercial video tape for Hi 8 (120-minute, ME) and ranked on thefollowing scales.

-   -   +3 dB or more:    -   +1 dB or more, less than +3 dB: ◯    -   less than 1 dB: x

When the determined value for output characteristics of a tape isgreater than that of the commercial video tape for Hi 8 (120 minutes ME)by +1 dB, the tape is deemed to be satisfactory as a VTR tape of digitalrecording mode.

(19) Output Characteristics of MP Tape

An MP tape was measured on C/N at 7 MHz±1 MHz using a commercial VTRdevice for Hi 8 (EV-BS3000, manufactured by Sony Corporation). The C/Nvalue thus obtained was compared to that of a commercial video tape forHi 8 (120 minutes, ME) and ranked on the following scales.

-   -   +3 dB or more:    -   +1 dB or more, less than +3 dB: ◯    -   less than 1 dB: x

Hereinbelow, the present invention will be illustrated by the followingexamples and comparative examples.

EXAMPLE 1

An A/B two-layered laminated film composed of the following polyester Aand polyester B was produced.

Polyester A:

Bis(hydroxymethyl) terephthalate was prepared from dimethylterephthalate and ethylene glycol using magnesium acetate as a catalystin the conventional manner. The bis(hydroxymethyl) terephthalate waspolymerized using germanium oxide as a catalyst, thereby obtainingpellets of polyethylene terephthalate which contained fine particlesgenerated from polymerization catalyst residues (i.e., inside particles)as little as possible (intrinsic viscosity: 0.65, melting point: 258°C., ΔTcg: 82° C., solution haze: 0.1%).

Polyester B:

Pellets of polyethylene terephthalate were prepared in the conventionalmanner in which 0.2% by weight of spherical crosslinked polystyreneparticles with an average particle diameter of 0.2 μm and 0.05% byweight of spherical crosslinked polystyrene particles with an averageparticle diameter of 0.3 μm (intrinsic viscosity: 0.62, melting point:258° C., ΔTcg: 80° C.).

The pellets prepared above were separately dried in vacuo at 180° C. for3 hours and then supplied to two extruders, respectively. The polyesterA and polyester B were molten at 290° C. and 285° C., respectively,filtered separately in the conventional manner, and then laminatedthrough a rectangular intermixing block (feed block) for two-layeredlamination. The thickness of each layer was adjusted by controlling therotation speed of a gear pump placed on each line to control theextruder output. Subsequently, the resultant laminate was caused toclosely contact with a casting drum of 25° C. in surface temperatureelectrostatically and then cooled to solidify, thereby giving anon-stretched film.

The non-stretched film was irradiated with UV light from the A layerside of the film at 25° C. for 1.5 second in an atmosphere of 1 atm withadjusting the irradiation length so that the energy density became 0.7J/cm². As the light source of UV light, a metal halide type UV lampmanufactured by Japan Storage Battery Co., Ltd. (Type A, MAN500L, 120W/cm, relative intensity of 270–300 nm of 38% (maximum light emissionintensity: 365 nm)) was used, and wavelengths less than 250 nm were cutoff.

The UV-irradiated non-stretched film was introduced to a transversestretching apparatus consisting of plural heated rolls and stretched 3.4times in a longitudinal direction in two steps at 95° C. Subsequently,the film was introduced in a stenter with the film end being crampedwith a clip and then stretched 4.2 times in a transverse direction at95° C. at a stretching rate of 250%/min. The biaxially stretched filmwas stretched again 1.2 times in a longitudinal direction at 120° C. Theresultant film was heat-treated at 210° C. for 5 seconds under constanttension, thereby producing a biaxially oriented polyester film having anA layer thickness of 6 μm and a whole thickness of 7 μm. The conditionsfor UV light irradiation are shown in Table 1, and the results ofevaluation of the film are shown in Tables 2 and 3.

A deposit layer of cobalt-nickel alloy (Ni: 20% by weight) was providedin a thickness of 200 nm on the surface of the A layer side of the filmusing a continuous vacuum deposition apparatus in the presence of atrace amount of oxygen. A carbon protective layer was formed on thedeposit layer surface in the conventional manner, and the resultant filmwas slit in a 8 mm width to obtain a pancake. From the pancake, a 200 mtape was taken and installed in a cassette as a cassette tape with aferromagnetic metal thin film layer (ME tape). The outputcharacteristics of the tape were evaluated and the results are shown inTable 3.

As shown in these tables, according to the present process utilizingirradiation with UV light, fine protrusions could be formed at highspeed and stably and a polyester film stable in quality was obtained.

EXAMPLES 2–4

Substantially the same manner as in Example 1 was performed, except thatthe conditions for UV light irradiation were changed to those shown inTable 1, thereby obtaining a biaxially oriented polyester film with athickness of 7 μm. In Example 2, a high pressure mercury lamp was used;in Example 3, the same high power type of metal halide lamp as that usedin Example 1 was used; and in Example 4, a low pressure mercury lamp wasused. In the low pressure mercury lamp, the relative intensity of lightwith wavelength of 270–300 nm was 5% and light with wavelengths of lessthan 250 nm were contained.

The film obtained in Example 2 using a high pressure mercury lamp as thelight source had the film characteristics according to the presentinvention, although the number of surface protrusions was somewhatreduced.

As shown in Example 3, in the case where the energy density isincreased, the surface fine protrusions can be formed satisfactorilyeven when the irradiation time is reduced to ⅓ of that employed inExample 1. In the film obtained in Example 4 using a low pressuremercury lamp with wavelengths of less than 250 nm, the surfacedeterioration was promoted, scratch resistance was somewhat reduced, andsurface roughness and number of protrusions were also reduced.

A ferromagnetic metal thin film layer was formed on the A layer-sidesurface of each of the films, thereby producing cassette tapes (MEtapes).

EXAMPLE 5

In this example, a mono-layered polyester film is exemplified. Pelletsof polyethylene terephthalate polymerized in the conventional mannerusing, as polymerization catalysts, 0.10% by weight of magnesiumacetate, 0.03% by weight of antimony trioxide, 0.35% by weight ofdimethyl phenylphosphate (intrinsic viscosity: 0.62, melting point: 258°C., ΔTcg: 51° C., solution haze: 0.7%) and pellets of polyester Aproduced in Example 1 were separately dried, mixed together at a ratioof 3:7, supplied to an extruder, extruded at 280° C. and then cooled,thereby obtaining an non-stretched film. The non-stretched film wasintroduced to a converging UV light irradiation device and irradiatedwith UV light from the both surfaces of the film for 1.0 second. The UVlamp used was of the same type as in Example 1. Subsequently, the filmwas stretched 3.5 times in a longitudinal direction at 90° C., and thenstretched 4.8 times in a transverse direction at 95° C. at a stretchingrate of 2000%/min. The resultant film was subjected to heat treatment at220° C. for 5 seconds under constant tension, thereby obtaining abiaxially oriented polyester film with a thickness of 7 μm.

In the resultant mono-layered film, fine protrusions were formed stablyand the surface quality was stable.

A ferromagnetic metal thin film layer was formed on the surface of theresultant film which did not contact with a casting drum in the samemanner as in Example 1, thereby producing a cassette tape (ME tape).

EXAMPLE 6

A biaxially oriented polyester film with a thickness of 7 μm wasproduced substantially the same manner as in Example 5, except that thepower of the lamp used for the UV light irradiation was increased toshorten the irradiation time. By increasing the power of the lamp,numerous fine surface protrusions could be formed at high speed by UVirradiation for as short as 0.5 second. In this case, a polyester filmwith stable surface quality could be formed continuously.

A ferromagnetic metal thin film layer was formed on the film in the samemanner as in Example 5, thereby obtaining a cassette tape (ME tape).

EXAMPLE 7

In this example, the polyester prepared in Example 5 was used aspolyester A, and polyester A containing no particles prepared in Example1 was used as polyester B. These polyesters were separately supplied totwo extruders and polyester A and polyester B were molten at 275° C. and280° C., respectively, extruded and laminated through a rectangularintermixing block (feed block) for three-layered lamination. Theresultant laminate was electrostatically caused to closely contact witha casting drum of 20° C. in surface temperature, thereby giving anon-stretched A/B/A three-layered film. The non-stretched film wasirradiation with the same light source of UV light as in Example 3 fromthe both surfaces of the film for 1.0 second. The resultant film wasstretched in a longitudinal direction, in a transverse direction and atransverse direction in turn in the same manner as in Example 1,heat-treated at 220° C. under constant tension for 10 seconds, wasrelaxed at a ratio of 2% in a transverse direction, thereby obtaining abiaxially oriented polyester film with an A layer thickness of 1.5 μmand a whole thickness of 7 μm.

A ferromagnetic metal thin film layer was formed on the surface of filmwhich did not contact with a casting drum in the same manner as inExample 1, thereby obtaining a cassette tape (ME tape).

EXAMPLE 8

An A/B two-layered lamination film was produced. Substantially the sameprocedure as in Example 1 was performed, except that the polyester Aprepared in Example 1 blended with 0.3% by weight of spherical silicaparticles with an average particle diameter of 0.03 μm was used as thepolymer for the A layer side. UV light was irradiated on the filmsurface of the A layer side for 2.0 seconds with adjusting theirradiation length so that energy density of 0.7 J/cm² was achieved. Inthis manner, a biaxially oriented laminated film with an A-layerthickness of 6 μm and a whole thickness of 7 μm was obtained.

A ferromagnetic metal thin film layer was formed on the A layer-sidesurface of the film in the same manner as in Example 1, therebyobtaining a cassette tape (ME tape).

EXAMPLE 9

A biaxially oriented polyester film and a cassette tape (ME tape) wereproduced substantially in the same manner as in Example 8, except thatthe particles to be blended with polyester A were alumina particles witha primary particle diameter of 0.02 μm and the conditions for UVirradiation were changed to those given in Table 1.

COMPARATIVE EXAMPLES 1 AND 2

In these comparative examples, an example of forming fine protrusions ona film surface by the conventional heat treatment method is illustrated.

Comparative Example 1 illustrates an A/B/A type of laminated film of thesame type as in Example 7, and Comparative Example 2 illustrates amono-layered film of the same type as in Example 5. Substantially thesame procedure as in Example 7 or 5 was performed, except that thenon-stretched film was not irradiated with UV light but was subjected toheat treatment instead. In this manner, biaxially oriented polyesterfilms with a thickness of 7 μm and cassette tapes (ME tapes) wereproduced.

For the A/B/A type of laminated film of Comparative Example 1, thenon-stretched film was heated with a radiation heater so that the filmsurface temperature became 185° C. and heat-treated at the sametemperature for 4 seconds.

For the mono-layered film of Comparative Example 2, the non-stretchedfilm was heated so that the film surface temperature became 150° C. andheat-treated at the same temperature for 20 seconds.

The results of evaluation of the resultant films are shown in Tables 2and 3. In these Comparative Examples, surface protrusions by the actionof crystallization could be formed on the film surface and the surfacessuperior in scratch resistance were achieved. However, the protrusionson the film surface were non-uniform in size, the surface roughness Raand the number of fine protrusions varied by more than 40%, andtherefore it was fail to produce films with stable quality.

COMPARATIVE EXAMPLES 3 AND 4

In these comparative examples, an example of forming fine protrusions onthe film surface by providing a thin film layer produced by theconventional particle addition method.

In Comparative Example 3, for the formation of an A/B type oftwo-layered laminated film, a non-stretched film was producedsubstantially in the same manner as in Example 8, except that sphericalsilica particles with an average particle diameter of 0.03 μm wereblended to the A layer polymer in an amount of 1.0% by weight (intrinsicviscosity: 0.65, melting point: 259° C., ΔTcg: 81° C.).

The non-stretched film was subjected to substantially the same procedureas in Example 1, except that irradiation with UV light was notperformed, thereby obtaining a biaxially stretched film. As shown inTable 2, since the amount of particles added was increased for the sakeof forming numerous fine protrusions, coarse protrusions were formed dueto aggregation of the particles, the number of protrusions wasdecreased, and scraping resistance of the film and outputcharacteristics of a magnetic tape using the film were deteriorated.

In Comparative Example 4, an A/B two-layered non-stretched film wasprepared in the same manner as in Example 1, and then monoaxiallystretched 3.4 times in a longitudinal direction. On the surface of the Alayer side, a coating solution with the following formula whichcontained water soluble polymers and fine particles with a particlediameter of 20 nm was coated so that the solid coating concentrationbecame 20 mg/m².

[Water soluble coating solution] Methyl cellulose 0.10% by weight Watersoluble polyester 0.3% by weight Aminoethysilane coupling agent 0.01% byweight Ultra-fine silica 0.03% by weight (average particle diameter: 20nm)

The resultant film was stretched 4.2 times in a transverse direction at110° C. with a tenter. The film was stretched 1.3 times in alongitudinal direction at 120° C. followed by heat treatment at 210° C.for 5 seconds under constant tension, thereby obtaining a biaxiallyoriented polyester film with a B layer thickness of 1 μm and a wholethickness of 7 μm. On the surface of the coating layer side, aferromagnetic thin film layer was formed in the same manner as inExample 1, thereby obtaining a cassette tape (ME tape).

In these comparative examples, although the film surface having fineprotrusions thereon could be achieved, no surface portion that did notcontain protrusions with 10 nm or larger in height could not be takenfrom the film surface in the AFM evaluation (field: 0.5 μm×0.5 μm).Since the surface protrusions formed by such coating method are mainlycomposed of protrusions generated from particles in the coating,particles are present on the film surface in an exposed state and,therefore, are likely to be drop off. As a result, a problem in filmsurface defect may occur that such particles are scraped during the filmformation and slitting process, causing scratches on the film surface.The scratch resistance of the film and the output characteristics of aME tape using the film were deteriorated.

TABLE 1 Light source of UV light Energy Relative intensity density oflight with 270– during Irradiation 300 nm wavelengths irradiation timeType (%) (J/cm²) (sec) Ex. 1 Metal halide, 38 0.7 1.5 parallel type Ex.2 High pressure 15 0.7 1.5 mercury, Parallel type Ex. 3 Metal halide, 382.7 0.5 parallel type Ex. 4 Low pressure 5 0.7 0.5 mercury, Paralleltype Ex. 5 Metal halide, 38 0.5 1.0 converging type Ex. 6 Metal halide,38 3.0 0.5 converging type Ex. 7 Metal halide, 38 2.0 1.0 convergingtype Ex. 8 Metal halide, 38 0.7 2.0 parallel type Ex. 9 Metal halide, 380.5 1.0 parallel type

TABLE 2 Surface portion containing no protrusion with 10 nm or more inheight Surface Difference in concentration Number of surface roughness(A) of carboxyl groups between protrusions with Ratio relative GrainFilm Ra surface part and inside of 3–5 nm in height to the whole sizeconfiguration (nm) Rz/Ra thin layer (×10⁴/mm²) surface (%) (nm²) Ex. 1A/B 1.8 6.5 0.006 4800 91 1080 Ex. 2 A/B 1.3 7.8 0.004 3300 88 600 Ex. 3A/B 2.3 6.0 0.012 6200 93 1480 Ex. 4 A/B 1.3 5.9 0.018 1800 90 320 Ex. 5Monolayer (A) 1.4 6.8 0.007 680 45 1900 Ex. 6 Monolayer (A) 1.8 12 0.0141100 66 3200 Ex. 7 A/B/A 1.1 9.1 0.014 1200 77 3800 Ex. 8 A/B 1.6 14.50.014 850 81 3500 Ex. 9 A/B 2.2 16 0.002 230 52 1200 C. Ex. 1 A/B/A 1821 0.000 86 10 1000 C. Ex. 2 Monolayer (A) 17 22 0.000 120 4 750 C. Ex.3 A/B 9.0 21 0.000 680 2.5 5800 C. Ex. 4 A/B 3.8 15 0.000 None 0 None

TABLE 3 Scratch Stability in surface Output characteristics resistanceprotrusion formation of ME tape Ex. 1 Excellent ◯

Ex. 2 Excellent ◯

Ex. 3 Excellent ◯

Ex. 4 Good ◯

Ex. 5 Excellent ◯

Ex. 6 Excellent ◯ ◯ Ex. 7 Excellent ◯

Ex. 8 Excellent ◯ ◯ Ex. 9 Excellent ◯ ◯ C. EX. 1 Good X X C. EX. 2 GoodX X C. EX. 3 Unacceptable Δ X C. EX. 4 Unacceptable X X

EXAMPLE 10

An A/B two-layered laminated film was prepared.

As polyester A, pellets of polyethylene terephthalate that waspolymerized in the conventional manner using 0.06% by weight ofmagnesium acetate, 0.008% by weight of antimony trioxide and 0.02% byweight of trimethyl phosphate as polymerization catalysts and blendedwith 0.1% by weight of spherical crosslinked polystyrene particles withan average particle diameter of 0.3 μm and 0.1% by weight of aluminaparticles with a primary particle diameter of 0.02 μm (intrinsicviscosity: 0.62, melting point: 259° C., ΔTcg: 81° C.) were used.

As polyester B, pellets of polyethylene terephthalate blended in theconventional manner with 0.5% by weight of spherical crosslinkedpolystyrene particles with an average particle diameter of 0.3 μm and0.07% by weight of spherical crosslinked polystyrene particles with anaverage particle diameter of 0.6 μm were used.

The pellets were separately dried in vacuo at 180° C. for 3 hours andthen supplied to two extruders, respectively. The polyester A andpolyester B were molten at 285° C., and then laminated through arectangular intermixing block (feed block) for two-layered lamination.The resultant laminate was caused to closely contact a casting drum withsurface temperature of 25° C. electrostatically and then cooled tosolidify, thereby giving a non-stretched film. The non-stretched filmwas irradiated with UV light on the surface of the A layer side of thefilm for 2.5 second with adjusting the irradiation length so that theenergy density became 3.0 J/cm². The resultant film was introduced to atransverse stretching apparatus consisting of plural heated rolls andstretched 3.4 times in a longitudinal direction in three steps at 90° C.The film was stretched 3.8 times in a transverse direction at 100° C. ata stretching rate of 2000%/min. The biaxially stretched film wasstretched again 1.6 times in a longitudinal direction at 130° C. Theresultant film was heat-treated at 210° C. for 5 seconds under constanttension, thereby producing a biaxially oriented polyester film with an Alayer thickness of 6.6 μm and a whole thickness of 7 μm. The conditionsfor UV light irradiation are shown in Table 4, and the results ofevaluation of the film are shown in Tables 5 and 6.

A magnetic coating and a non-magnetic coating having the compositionsgiven below were coated in an overlaying manner on the surface of the Alayer side of the film using an extrusion coater (the upper layer is themagnetic coating in a coating thickness of 0.2 μm; the lower layer isthe non-magnetic coating in a coating thickness of 1.8 μm), magneticallyoriented, and then dried. On the surface of the opposite side, a backcoat layer having the composition given below was formed in theconventional manner. The resultant film was calendered using a smalltest calender (steel roll/steel roll, 5 steps) at 85° C. at a linearpressure of 200 kg/cm, and then cured at 60° C. for 48 hours. The filmdestined to form into tapes was slit to thereby produce a pancake. Fromthe pancake, a tape was taken and installed in a cassette, therebyproducing a cassette tape having a metal-coated magnetic layer.

[Composition of magnetic coating] Ferromagnetic metal powder 100 partsby weight Sodium sulfonate-modified vinyl chloride 10 parts by weightcopolymer Sodium sulfonate-modified polyurethane 10 parts by weightPolyisocyanate 5 parts by weight Stearic acid 1.5 parts by weight Oleicacid 1 part by weight Carbon black 1 part by weight Alumina 10 parts byweight Methyl ethyl ketone 75 parts by weight Cyclohexanone 75 parts byweight Toluene 75 parts by weight [Composition of non-magnetic lowerlayer] Titanium oxide 100 parts by weight Carbon black 10 parts byweight Sodium sulfonate-modified vinyl chloride 10 parts by weightcopolymer Sodium sulfonate-modified polyurethane 10 parts by weightMethyl ethyl ketone 30 parts by weight Methyl isobutyl ketone 30 partsby weight Toluene 30 parts by weight [Composition of back coat layer]Carbon black 95 parts by weight (average particle diameter: 20 nm)Carbon black 5 parts by weight (average particle diameter: 280 nm)α-Alumina 0.1 part by weight Zinc oxide 0.3 part by weight Sodiumsulfonate-modified vinyl chloride 30 parts by weight copolymer Sodiumsulfonate-modified polyurethane 20 parts by weight Methyl ethyl ketone300 parts by weight Cyclohexanone 200 parts by weight Toluene 100 partsby weight

As shown in the tables, according to the present method usingirradiation with UV light, fine protrusions could be formed at highspeed and stably, a polyester film superior in scratch resistance wasobtained and, when a metal-coated magnetic layer was provided on the Alayer side surface and a back coat layer was provided on the B layerside surface to produce a magnetic tape (ME tape), outputcharacteristics of the magnetic tape became good.

EXAMPLE 11

An A/B/A three-layered laminated film was prepared.

As the polymer for the A layer (polyester A), polyethylene terephthalatepolymerized in the conventional manner (polymerization catalysts: 0.20%by weight of magnesium acetate, 0.03% by weight of antimony trioxide and0.20% by weight of dimethyl phenylphosphate as a phosphorus compound)was used (intrinsic viscosity: 0.63, melting point: 258° C., ΔTcg: 68°C., solution haze: 1.8%). As polyester B, polyester pellets withoutparticles were used. The pellets were separately dried, and supplied totwo extruders, respectively. The polyester A and polyester B were moltedat 275° C. and 285° C., respectively, and laminated through arectangular intermixing block (feed block) for three-layered lamination.The resultant laminate was caused to closely contact with a casting drumand cooled to solidify, thereby producing an A/B/A three-layerednon-stretched film.

The non-stretched film was irradiation with UV light from the bothsurfaces under the conditions given in Table 4. The resultant film wasstretched and heat-treated in the same manner as in Example 10, therebyobtaining a biaxially oriented polyester film with an A layer thicknessof 1 μm and a whole thickness of 7 μm.

A magnetic coating and a non-magnetic coating were coated on the surfacewhich did not contact with the casting drum, magnetically oriented anddried in the same manner as in Example 10. A back coat layer was formedon the opposite side of the film, calendered and then cured, therebyobtaining a cassette tape (MP tape) having a metal-coated magnetic layerthereon. By employing the UV irradiation conditions as shown in Table 4,although the diameters of the surface protrusions produced by UVirradiation became large, the surface protrusions could be formed athigh speed and, therefore, a polyester film superior in scratchresistance and output characteristics in the form of a MP tape could beformed continuously.

COMPARATIVE EXAMPLE 5

A biaxially oriented polyester film and a cassette tape (MP tape) with ametal-coated magnetic layer were prepared substantially in the samemanner as in Example 11, except that the non-stretched film was notirradiated with UV light. Since UV light was not irradiated, suchproblems occurred that fine protrusions could not be formed and frictionbetween the film and the rolls during the film formation/slittingprocess was increased, causing scratches on the film surface. Thescratch resistance of the film and output characteristics of the MP tapewere deteriorated.

TABLE 4 Light source of UV light Relative intensity of Energy light withdensity 270–300 during Irradiation nm wavelengths irradiation time Type(%) (J/cm²) (sec) Ex. 10 Metal halide, 38 3.0 2.5 parallel type Ex. 11Metal halide, 38 3.5 2.0 converging type

TABLE 5 Surface portion containing no Protrusion with 10 nm or more inheight Surface Difference in concentration Number of surface roughness(A) of carboxyl groups between protrusions with Ratio relative GrainFilm Ra surface part and inside of 3–5 nm in height to the whole sizeconfiguration (nm) Rz/Ra thin layer (×10⁴/mm²) surface (%) (nm²) Ex. 10A/B 8.5 14.1 0.017 120 25 800 Ex. 11 A/B/A 19.2 13.7 0.018 500 30 4300C. Ex. 5 A/B/A 2.2 23 0.000 1 55 0.2

TABLE 6 Scratch Stability in surface Output characteristics resistanceprotrusion formation of ME tape Ex. 10 Excellent ◯

Ex. 11 Excellent ◯ ◯ C. Ex. 5 Unacceptable X X

INDUSTRIAL APPLICABILITY

According to the production process for a polyester film of the presentinvention, fine protrusions can be formed on a surface of a polyesterfilm extremely readily, a polyester film can be produced stably whichexhibits good wear resistance and running durability and good outputcharacteristics when used as a base film for a magnetic tape, and thefilm can be formed at high speed advantageously. Therefore, the presentprocess is quite useful for production of polyester films in industrialscale.

The polyester film of the present invention produced by the presentprocess is quite useful as a base film for a magnetic recordingmaterial, particularly for a magnetic recording medium having aferromagnetic metal thin film layer thereon. In addition, the presentpolyester film can also be widely and effectively utilized for variousfilm applications such as thermal transfer ribbons, heat-sensitivemimeograph stencils and electric capacitors.

1. A polyester film characterized in that a film surface portioncontaining no protrusion with 10 nm or more in height constitutes atleast 5% of the whole film surface; and the number of protrusions withnot less than 3 nm and less than 5 nm in protrusion height in the filmsurface portion is not less than 1×10⁶/mm² and less than 1×10⁹/mm².
 2. Apolyester film characterized in that fine protrusions are present on atleast one surface of the film; the ratio of 10-point average roughnessRz to center line average roughness Ra (Rz/Ra) on the surface is lessthan 20; the concentration of carboxyl groups in the surface layer partof a thin layer which provides the surface is greater than that in theinside of the thin layer; a film surface portion that contains noprotrusion with 10 nm or more in height constitutes at least 5% of thewhole film surface; and the number of protrusions with not less than 3nm and less than 5 nm in protrusion height in the film surface portionis not less than 1×10⁶/mm² and less than 1×10⁹/mm².
 3. The polyesterfilm according to claim 2, wherein the difference in concentration ofcarboxyl groups between the surface layer part of a thin layer whichprovides the surface having fine protrusions thereon and the inside ofthe thin layer is not less than 0.001.
 4. The polyester film accordingto claim 1 or 2, wherein, in the film surface portion containing noprotrusion with 10 nm or more in height, the grain size at theprotrusion height threshold value of 3 nm is not less than 1 nm² andless than 5000 nm².
 5. The polyester film according to claim 1 or 2,wherein particles having a particle diameter in the form of monodisperseparticles and/or a primary particle diameter in the form of aggregateparticles of not less than 1 nm and less than 300 nm are contained in anamount of not smaller than 0.01% by weight and smaller than 1% byweight.
 6. The polyester film according to claim 1 or 2, wherein thefilm is a laminated film that has an A layer mainly composed ofpolyester A laminated on at least one surface of a B layer mainlycomposed of polyester B.
 7. A magnetic recording medium comprising amagnetic layer provided on a surface having fine protrusions thereon ofa polyester film as recited in claim 1 or
 2. 8. The magnetic recordingmedium according to claim 7, wherein the magnetic recording medium is acassette-type magnetic tape of a digital recording mode.
 9. The magneticrecording medium according to claim 7, wherein the magnetic layer is aferromagnetic metal thin film layer.